Insecticides are natural or synthetic chemicals used to manage insects pests; they are important for disease control and providing food and fiber for a growing world population. Insect control with chemicals began about 2,000 years ago with the use of natural products, whereas the age of synthetic insecticides began with the introduction of dichlorodiphenyl trichloroethane (DDT) in the 1940s. Here the major classes of insecticides are covered, and important example compounds provided. The discussion is organized topically by mode of action/target tissue. In addition to synthetic materials, natural products are addressed because their use is increasing in "organic farming." This entry then explores the development of plants or viruses genetically engineered to produce insect-selective toxins . The harmful effects of insecticides on humans will also be discussed, as well as some nonchemical control techniques.
Compounds Affecting Nerves
The pyrethroids are composed of natural pyrethrins, which are isolated from chrysanthemum flowers, as well as newer synthetic materials. Older pyrethroids (e.g., pyrethrins and tetramethrin) degrade too rapidly in the environment to be used in agriculture. They are used in buildings, and because of their general safety, they are even applied to humans to control lice. Newer pyrethroids have greater chemical stability (e.g., permethrin and deltamethrin), which allows their use on many types of field crops. Another important use of permethrin is application to mosquito netting. Intoxication by pyrethroids develops rapidly (in 1 to 2 minutes), and involves a rapid loss of normal posture and movement called "knockdown." Pyrethroids affect nerve impulse generation throughout the entire nervous system. Multiple nerve impulses occur when only a single one was expected, and there is an increased release of chemical neurotransmitters as well. These actions result in convulsions, prostration, and death.
Sabadilla, an extract from the seeds of a tropical lily, is used in home gardens and organic farming operations. It degrades rapidly in the environment, and causes signs of intoxication, and has a mode of action similar to that of pyrethroids. Sabadilla extract has low toxicity to mammals.
The tobacco compound nicotine has been used as an insecticide for over 200 years. It is especially effective against sucking insects, such as aphids, and has excellent contact activity . Related compounds are neonicotinoids (e.g., imidacloprid), which have similar insecticidal activity, but are less toxic to mammals. Nicotine and imidacloprid mimic the action of acetylcholine , which is the major excitatory neurotransmitter in an insect's central nervous system. The action of acetylcholine is stopped by the enzyme acetylcholinesterase, which rapidly breaks down acetylcholine. Nicotine and imidacloprid are also neuroexcitatory, but do so persistently, since they are not affected by acetylcholinesterase. Overstimulation of the nervous system often leads to convulsions, paralysis, and death.
The organophosphorus (OP) and carbamate insecticides are used to control a wide variety of insect pests. The acute toxicity of the OPs and carbamates varies, and many of them have high mammalian toxicity . These compounds react chemically with the active site of acetylcholinesterase, producing a blocked enzyme that cannot degrade acetylcholine. The concentration of acetylcholine then builds up and hyperexcitation occurs. The signs of intoxication include restlessness, tremors, convulsions, and paralysis. Blockage of acetylcholinesterase by OPs is persistent, and recovery of the enzyme takes many hours or even days. The mode of action of the carbamates
is similar, except that enzyme blockage is less stable and recovers in a matter of minutes. Among insects, carbamates are particularly toxic to hymenoptera, such as honeybees.
Organochlorines represent one of the oldest groups of synthetic insecticides, with only biodegradable materials such as lindane and endosulfan still used in pest control. High mammalian toxicity was common with organochlorines, but a newer compound, fipronil, has improved selective toxicity toward a variety of insect pests. These insecticides cause hyperexcitability and convulsions by blocking the inhibitory neurotransmitter γ ‐aminobutyric acid (GABA). Normally, GABA has a dampening effect that maintains normal nerve activity. Blocking the effects of GABA removes inhibition, leading to hyperexcitation of the nervous system and convulsions.
Deet is an important insect repellant. This compound is applied to skin or clothing, and repels biting flies (e.g., blackflies and mosquitoes). Deet acts on the sensory nerves, causing insects to avoid treated surfaces.
Compounds Affecting Muscles
Ryania consists of the powdered stem of the tropical shrub, Ryania speciosa. The extract contains ryanodine and related compounds, and has a low toxicity to mammals. The powder is used as a stomach poison on vegetables and fruit. Ryanodine induces paralysis in insects by direct action on the muscles, resulting in sustained contraction and paralysis.
Avermectins are a group of closely related compounds isolated from the fungus Streptomyces avermitilis that are used to control the parasites of humans and animals, as well as arthropod pests in crops. They have fairly high mammalian toxicity, but their movement into treated leaves, oral activity against insect pests, and rapid breakdown in sunlight are all favorable properties. In insects and worms poisoned by avermectin, inactivity and flaccid paralysis occur from its relaxing effect on muscles.
Compounds Disrupting Energy Metabolism
These compounds vary, from the natural product rotenone (from Derris or Lonchocarpus root, used to control vegetable and fruit insects) to the synthetics sulfluramid and hydramethylnon (used to control mites and cockroaches). Interestingly, the highest acute toxicity to mammals is caused by the natural product rotenone. These compounds affect the production of adenosine triphosphate (ATP) , the energy storage molecule of the cell that is produced by mitochondria, the "powerhouse" of the cell. The disruption of energy metabolism and the subsequent loss of ATP result in a slowly developing toxicity, and the effects of all these compounds include inactivity, paralysis, and death.
Insect Growth Regulators
Insects exposed to diflubenzuron and related compounds are unable to form normal cuticle (skin) because their ability to synthesize it is lost. Thus, the cuticle becomes thin and brittle, and is unable to support the insect or to withstand molting, a process requiring the shedding of the old cuticle, as in metamorphosis. Diflubenzuron and other chitin synthesis inhibitors have extremely low mammalian toxicity and are used against termites.
Methoprene and fenoxycarb mimic the action of insect juvenile hormone in molting and reproduction, and have low toxicity to mammals. Exposure at molting produces deformed insects having mixed larval/pupal or larval/adult morphologies, and they disrupt reproductive physiology in adults to effectively serve as a method of birth control.
Tebufenozide acts by mimicking the effects of the insect hormone ecdysone, which along with juvenile hormone, controls the initiation of a molt. Exposure to tebufenozide induces a premature molt that traps the insect in its old cuticle. This compound is especially effective against caterpillars.
Toxins from Bacillus thuringiensis
The bacterium Bacillus thuringiensis forms an internal crystal that contains a number of insecticidal protein toxins. When eaten by the insect, the crystal dissolves in the midgut, the toxin mixture is released, and the proteins are cleaved into active forms. The toxins bind specifically to midgut cells and assemble a pore that leads to disintegration of the cells, gut paralysis, and death. B. thuringiensis strains have toxins specific for caterpillars, beetles, or flies. They have little or no effect on mammals.
Human Toxicity of Insecticides
In mammals DDT and related organochlorines have effects on the endocrine system, including the disruption of thyroid hormone synthesis and mimicking of the effects of estrogen . Liver cancer has also been observed in mice exposed to these substances, and there has been one claimed association between exposure to DDT and breast cancer. Epidemiological studies show a consistent connection between exposure to pesticides and the occurrence of Parkinson's disease in rural populations. A well-documented effect of some OPs is organophosphorus-induced delayed neuropathy , a slowly developing degeneration of the leg nerves that results in irreversible limping. A specific hazard of pyrethroids is paresthesia, a tingling or burning sensation in exposed skin.
Nonchemical Control Methods
There is considerable interest in developing genetically enhanced, insect-specific viruses or crop plants that would replace conventional chemical insecticides. Corn, cotton, and potatoes have been engineered to express B. thuringiensis toxins to control chewing insects. Although this approach has worked effectively for controlling some pests, others not targeted by the B. thuringiensis toxin must be controlled by other means.
Sex pheromones, chemicals that attract one sex of an insect to the other, also have uses in pest control. They are often utilized with traps to monitor the number of pest insects in an area, and when applied in the field at higher levels, they can disrupt reproduction or egg laying.
Biological control involves the introduction of predators and parasitoids to attack pests. The extent of control using this technique varies and can be quite good in some cases, but unforeseen ecological impacts occur when imported species attack nontarget organisms.
Chemical insecticides remain an important tool for managing insect pests of humans, animals, and food and fiber crops. Compounds that are persistent in the environment are no longer used, and the amounts sprayed have dropped from kilograms per acre to grams per acre of active ingredient. Future compounds and technologies will seek to maintain high levels of effectiveness with a reduced impact on the environment and human health.
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